The present invention relates generally to multi-conductor cables for the transmission of electric power. More particularly, the present invention relates to flat multi-conductor power cables which are thin enough to be laid underneath a carpet. The present invention also relates to a method and apparatus for making such cables.
In recent years, the use of such flat power cables has become popular in certain applications. For example, flat power cable can be used to supply power to outlets which are located in the middle of a room, i.e. away from walls, by laying the flat cable under a carpet. Hence the term "undercarpet cable." Naturally, placing the power cable under the carpet is safer than running a cord across the floor. Likewise, installing undercarpet cable is more economical than installing conduits within the floor, especially when remodeling an older building.
Typically, undercarpet cable is made by laminating three or five spaced apart flat copper conductors between a top and bottom sheet of insulating material, such as a polyester. An adhesive is applied to the polyester sheets and the two sheets are then pressed onto the conductors with a certain amount of heat and pressure. Unfortunately, this lamination process is relatively slow and therefore inefficient.
Undercarpet cable is generally used in a relatively harsh environment with sever demands being made on the insulating material and the integrity of the cable. For one thing, undercarpet cable is often laid on top of a concrete floor. Concrete has a tendency to retain a certain amount of moisture which can be deleterious to the insulation layer and particularly harmful if the moisture penetrates through to the conductors. To make matters worse, the moisture emitted from concrete normally has a relatively high pH, sometimes in the order of 11 or more. Such high alkalinity can lead to erosion of certain insulating materials.
Another factor which places demands on undercarpet cable is that the cable will be subject to a certain amount of abuse caused by the traffic passing over it. In particular, when an object such as a person's foot or the wheel of a cart pushes down on the carpet above the cable, the cable may be subject to abrasion if there is relative motion between the cable and the carpet or the floor. In addition, the insulation must be resistant to penetration by particles which are pushed into the cable by the weight of an object on top of the cable. These problems are exacerbated when laying the undercarpet cable on concrete floors which normally have an abundance of particles to cause such abrasion and penetration.
Still another factor which places demands on undercarpet cable is the fact that it is made to lay horizontal and flat. As a result water is more likely to accumulate on top of the flat cable than it would on a conventional round cable which does not present a surface upon which the water can accumulate. Naturally, such accumulation could be harmful, especially if the cable had been penetrated by a particle. Accordingly, penetration resistance is even more important.
Yet another factor which places demands on undercarpet cable is the fact that, because the cable is flat, the only way to make it turn corners is to fold the cable over to thereby produce the bends needed. As a result, the cable must be amenable to folding. That is, the combination of the conductor and insulation must be relatively flexible and able to fold sharply. In addition, because such folding results in the top and bottom of the cable being reversed, both sides of the cable must be equally suited to be on the bottom and the top. In other words the abrasion resistance cannot be improved by simply putting on a thicker bottom layer of insulation. The insulation would have to be thicker on both sides since the top and bottom switch upon folding. U.S. Pat. No. 4,283,593 teaches a method of folding undercarpet cable which maintains the top and bottom as such. However, this method produces a fold which is 50% thicker than a conventional fold.
Still yet another factor which places demands on undercarpet cable is the fact that the cable cannot be too thick. That is, to avoid detection above the carpet, the cable must be relatively thin. The thickness of the conductors will have a certain minimum thickness in order to carry the desired current. As a result, the insulating layers should be as thin as possible. Naturally, this requirement of thinness conflicts with the need for abrasion and penetration resistance.
One method which has been implemented to lessen these demands on undercarpet cable has been to include a protective sheet below and above the cable when it is laid. For example, 0.25 mm sheets of polyvinyl chloride have been laid beneath and above the undercarpet cable. Sheets of galvanized steel have also been used to overlay the cable and protect it from the effects of traffic. See also U.S. Pat. No. 4,283,593 which shows an undercarpet cable which has extra metal and plastic sheaths attached directly to the cable. Naturally, either of these methods increases the cost of installing undercarpet cable. In addition, neither completely solves the problems associated with moisture.